Mass spectrometry



United States atent fifice 3,016,459 Patented Jan. 9, 1962 3,016,459 MASS SPECTROMETRY Lewis Friedman, Patchogue, N.Y., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Dec. 16, 1959, Ser. No. 860,051

, 2 Clairns. (CL 250-413) The present invention relates to improvements in mass spectrometers, and more particularly, to a method for improving the resolution qualities of mass spectrometers. Mass spectrometers are utilized to obtain relative mass measurements of ions which are made to pass through the deflecting influences of an electrostatic and magnetic field. With a deflection type of instrument, employing a combination of electrostatic and magnetic sector fields, ions of different mass in the incident ion beam will focus at different spatial positions on the instruments focal plane, producing the mass spectrum. The electrostatic and magnetic fields are combined to produce double focusing; that is, all ions of the same imass entering the instrument within a small divergence angle and which have an energy different from the mean energy by a. small amount will focus. If a photographic plate is placed in the focal plane, a mass spectrum will appear, and distance along the plate may be correlated with mass. It is thus apparent that the degree of resolution and thereby degree of accuracy of measurement obtainable with such an instrument will depend upon how closely the electorstatic and magnetic resolving fields can be maintained, especially when measurements with an accuracy of a few parts per million are to be made or where low magnetic fields and low accelerating voltages are used.

It is generally understood that the degree of resolution obtainable in any given machine of this type decreases with use. All available evidence appears to indicate that the trouble is caused by the deposition on the interior walls and parts of the vacuum chamber of the spectrometer of a film of non-conducting material and the subsequent building up of a charge on the film. The nonconducting material is assumed to be primarily hydrocarbons or organic compounds obtained either by back diffusion of vapors from the oil diffusion pumps or from samples investigated in the mass spectrometer. Such a charge alters the path of the particles under study and thus affects the results of the work. Usually, after a period of time when the degree of resolution has dropped to a point below acceptable values, the spectrometer is disassembled and thoroughly cleaned on its inner surfaces. In some situations, the films are removed by heating the vacuum chamber without disassembling it and evaporating the films. However, this is not wholly effective and cannot be used on all spectrometers. Both of the above mentioned cleaning methods require that the spectrometer be taken out of service, at least for a short time; and neither of them prevents a drop in resolution with use.

It is thus a first object of this invention to provide for the prevention or continual correction of the condition described above and to greatly extend the time during which a spectrometer may be operated before it must be disassembled for cleaning. As a further object it involves the establishment of conducting paths on the surface of the insulating film building up as noted above so that any charge thereon may be dissipated.

In a first preferred aspect of this invention a radioactive gas such as hydrogen tritide (HT) or carbon-l4 methane (C I-l is supplied to the spectrometer along with the sample to be investigated. This gas continually emits beta particles and associated ionizing radiation which establishes leakage paths on the surface of the noncond-ucting film and thereby provides the means for the rapid dissipation of the charges thereon. Tests of this method show that the time between physical cleanings can be extended by a factor of from two to four, and that the rate of deterioration of the resolution of the machine is greatly reduced.

For example, in one series of tests it was found that the addition of small amounts of radioactive carbon-14 methane (C H to water vapor being measured the resolution of the instrument was found to increase from one part in 27,400 to one part in 33,500.

The principle of this invention was shown in a series of tests in which a mass comparison of He and D showed a resolution of 19,500 followed by a comparison of HD and He in which a resolution of 21,700 was obtained. Then radioactive HT was compared with He and the resolution was increased to 25,900. The machine was then pumped out and left unused for two days and He and D; were compared with a resolution of 18,900 thereby indicating a lack of persistence in the effect of the radioactive gas, hence, that this effect was due solely to the presence of the radioactive gas and not due to any chemical reaction with the insulating film building up. This was confirmed by a comparison of HT and He at a resolution of 26,000.

As would be expected, this effect is evident only where a very high resolution instrument is being used, or where low accelerating voltages are present, so that the relatively low static charge built up, i.e., of the order of one volt or less, would have a significant effect on the results. A low voltage instrument is the cycloidal focusing mass spectrometer developed by Consolidated Electrodynamic Corporation, while a high voltage high resolution instrument is one commonly known as the mass synchrometer, both found to be suitable for the practice of this invention.

In utilizing this principle, it is thus seen that a small amount of radioactivity introduced with each sample or group of samples being measured will effectively increase the resolution sensitivity of the instrument and extend the period of use before dismantling and cleaning is required.

In a second preferred embodiment of this invention, a practically chemically inert beta-emitting isotope, such as cadmium-109, or silver-110, is plated on the interior of the metal shell or charged chamber of the spectrometer, in a thin layer. Radiations from the plated isotope form leakage paths from the surface of the non-conducting film to the metal shell of the spectrometer either over the surface of the film as in the radioactive gas method described above, or through the film, so that the charges on the surface of the film are dissipated. The effect of the coating of course'would linger to an extent depending upon the half-life of the radioactive isotope, as is understood in the art.

It will be noted that all of the examples given involve the use of beta emitters. Beta rays are very suitable for use in this invention because of the great availability of materials with such decay in a range of masses wherefrom such materials must be selected. Also, because of the short range of travel of these rays there is no significant shielding problem. On the other hand, gamma decay materials are generally not suitable for several reasons. There is a reduction in effectiveness on a surface path because of the great penetration and, because of the energies involved, problems in shielding are presented. With regard to alpha radiation, presumably, materials undergoing such decay would be useful, except that under certain circumstances the penetrating power of the rays is so small that an adverse effect on resolution might be found. The unstable materials with such decay are mostly found in the heavy end of the periodic table to begin with so that there is only a very limited choice of materials available.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

I claim:

1. A method for improving the resolution qualities of a mass spectrometer in which non-conducting films are built up on the walls thereof which comprises the step of adding a small amount of a beta emitting material to each sample being tested for discharging before said testing, and thereby creating a leakage path in said film so as to prevent the accumulation of, a static electric charge on the interior walls of said spectrometer which would adversely afiect the resolution characteristics thereof.

2. A method for improving the resolution qualities of a mass spectrometer in which non-conducting films are built up on the walls thereof which comprises the step of adding a small amount of a beta emitting material selected from a group consisting of hydrogen tritide and carbon- 14 methane to each sample being tested for discharging before said testing, and thereby creating a leakage path in said film so as to prevent the accumulation of, a static electric charge on the interior walls of said spectrometer which would adversely affect the resolution characteristics thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,048,490 Bilstein July 21, 1936 2,264,683 Smith Dec. 2, 1941 2,378,936 Langmuir June 26, 1945 2,599,166 Dempster June 3, 1952 2,806,143 Carter Sept. 10, 1957 2,848,620 Backus Aug. 19, 1958 

1. A METHOD FOR IMPROVING THE RESOLUTION QUALITIES OF A MASS SPECTROMETER IN WHICH NON-CONDUCTING FILMS ARE BUILT UP ON THE WALLS THEREOF WHICH COMPRISES THE STEP OF ADDING A SMALL AMOUT OF A BETA EMMITTING MATERIAL TO EACH SAMPLE BEING TESTED FOR DISCHARGING BEFORE SAID TESTING, AND THEREBY CREATING A LEAKAGE PATH IN SAID FILM SO AS TO PREVENT THE ACCUMULATION OF, A STATIC ELECTRIC CHARGE ON THE INTERIOR WALL OF SAID SPECTROMETER WHICH WOULD ADVERSELY AFFECT THE RESOLUTION CHARACTERISTICS THEREOF. 